Platelet adhesion to vascular subendothelium has been shown to be a crucial initial step in the primary hemostatic reaction. The bleeding diatheses of patients with disorders such as von Willebrand's disease and Bernard-Soulier syndrome are attributable to defects in this interaction between platelets and exposed vascular subendothelium at sites of trauma. By applying the perfusion technique of Baumgartner to human arterial segments, this project attempts to better define the biochemical interactions between platelets and macromolecular constituents of vascular subendothelium which underlie this vital component of primary hemostasis. To this end, platelets and plasma from patients with the above disorders, as well as other conditions associated with impaired hemostasis, will be studied in order to assess their competence in supporting platelet adherence to the vascular segments. Subsequently, experimental manipulations of the platelets, plasma and subendothelium will be employed to approach the problem.